0-3 gauge and negative PSI

I don't think it's your water pump that is making the pressure go negative. Rather, it is just the collapse of steam after the boiler shuts off that makes your pressure go negative. This is normal operation.

I haven't noticed my gauge having any trouble with this brief negative pressure, but if you're concerned you can put a ball valve under the gauge and only open it when you want to see the pressure.

Dwyer gauges including their Magnahelic can definitely handle it but one of mine is very similar to yours and it's OK.

You might destroy the gauge, I can't vouch for that, but I don't think it will cause you any issue beyond that

Honestly, your gauge already looks a little jacked up 😅 Is the needle no longer pointing where it used to?

There's a deeper problem here. If you are managing to get a vacuum, your vents are not opening properly.

Buy a 30-0-30 compound gauge if you want. The vacuum will probably damage the gauge.

You could easily install a steam vacuum breaker on the system (Tunstall sells those) or use a swing check valve. Pipe it above the boiler water line so it will let air and close under pressure.

Is your HW loop pumping boiler water or are you pumping through a tankless coil or a heat exchanger.

Hmm… OK. Maybe the reason I never see a vacuum for Cedric is that the main (and only) vents (Gorton #2 and Hoffman 75) are right at the boiler (on the dry returns) and never close. Pressure gauge is on the header, which connects though the Differential Loop to the dry returns…

Excuse me if this was asked and answered and I missed it, but what brand and model of radiator vents do you have? I read an article by Dan Halohan about the old tyme vents from the coal days that tended to stay shut but it didn't matter because with coal boilers they kept them going at a low rate all day.

All Heat Timer Varivalves here as well. Regular Honeywell Pressuretrol set at 1.5 psi cut out, 0.5 psi cut in. I have heard one inhale air noisily occasionally some time after a heating cycle so I think they can stick sometimes and not open a the same temperature they closed at. Haven't heard it happen this season.

I think what could be happening is that your hydronic loop with the circulator pump could rapidly remove heat from the boiling water and therefore the steamchest, causing rapid condensation of the steamchest, creating a vacuum (due to the fact that liquid water is more dense than the steam or approx. 1,600 x lower specific volume than the steam) before the radiators and the mains have had a chance to cool off, therefore the vents are still closed. A lot less moles of steam suddenly occupying almost the same volume (very slightly less due to the volume of the extra condensate) definitely results in a large pressure drop, easily going into a vacuum.

Yeah, 2-0-2 PSI seems odd.

Does your pressure really go that high ?

My boiler never goes over 2 inches of water column (0.072 PSIG). Maybe you can find a Compound Gauge with a narrower range than 30 PSI - 0 - 30 inHg.

Or even with the 2202 0-2 PSI I'd be very surprised if the vacuum would damage the Magnahelic.

https://www.mmcontrol.com/Dwyer/pdf/02-Pressure-2020.pdf

Well I guess it matters to the OP. I think that it can happen with a cold basement. I just heard one of my radiators inhale a bit a very short time after the burner cycle ended. It's 3.2 F here in NJ right now.

Where it matters is that having a hydronic loop that rapidly withdraws heat from the boiler, and isn't prevented from doing so right after a boiler cycle has completed (maybe there is a temperature switch lockout you can install?) will make the phenomena of getting a vacuum before vents can cool enough to open back up, much more likely to happen and deeper in vacuum.

It is making me wonder now. I have a 0 to 3 psi gauge which I don't remember the brand name right now, but it has a zero peg. I wouldn't know if it is trying to go below zero, or if that peg prevents damage to the gauge from a vacuum. I need to find out. It is true that my cut out that was pretty solid at 1.5 psi has drifted to close to 2 psi. I'm thinking about buying a new gauge to see if it is the gauge. Can't set the Pressuretrol any lower than it is.

If you read what I said, I didn't say that you can't get a small vacuum under certain circumstances, but it is WORSE with a hydronic loop that turns on at that time, and some types of radiator vents may be more susceptible to hysteresis (Dan Halohan's old tyme radiator vents from the coal era). Also, a cold basement absolutely can cool the boiler and near boiler piping off that much faster (basic heat transfer), which doesn't have any effect on the radiator vents.

@Captain Who

" or if that peg prevents damage to the gauge from a vacuum. "

I think you will find in some situations the peg causes the damage. Since it jams the needle causing stress on the linkage and the Bourdon tube, that the gauge was not designed to withstand.

I would not worry about the vacuum, just get a suitable gauge.

I guess it could be possible. Does your hot water loop have a bypass loop on it, or does it pull directly from the boiler water?

And is there a vertical drop from the boiler to the water circulator pump?

OK sorry for assuming it mattered to you 😃. Anyway, I have been waiting for you to answer @EBEBRATT-Ed 's question above:

"Is your HW loop pumping boiler water or are you pumping through a tankless coil or a heat exchanger."

Edit to add - although I guess you just did, indirectly. Ask the pros how to prevent this cycling when boiler water is basically at Tsat.

Edit to also add - It's been a lot more than 30 years for me lol, but you are on the right track. Tsat is lower at lower pressure, therefore easier to get vapor in your hydronic line.

I've been thinking about getting a digital manometer for testing things like automotive PCV system and the like, and they do vacuum and pressure. I definitely make sure I don't have the pressure regulator on the compressor set too high and poke the long cable tie through the pigtail into the boiler first to remove any major restriction. Then I barely see the needle move on the gauge when I blast it with a shot of compressed air.

At least 3 of us have suggested compound gauges. Of course, a hard vacuum will eventually damage a low-pressure gauge they are pretty sensitive.

The vacuum will allow the steam to boil at a low temp when the boiler shuts down.

If you don't like the vacuum noise or the rads inhaling put a check valve or a vacuum breaker on the boiler.

So looks like about 3 in. H2O of vacuum? That'd be 0.108 psi of vacuum. Hard to believe that would damage a 0 to 3 psi low pressure gauge. However, how low could it go if one had a hydronic loop circulating through a heat exchanger in an upstairs room, pulling heat out of the steamchest rapidly, condensing that steam rapidly?

Ha. I don't know what crazy science you two are using but if there is a rapid condensation (due to rapid heat extraction) in a closed system (vents still closed), that means fewer moles of vapor occupying roughly the same volume as before. Instead of imagining science look it up. That results in lower vapor pressure. That vapor pressure could easily go into a vacuum and a lower vacuum than would normally be observed.

Make it stop. I've entered the Twilight Zone. 🤣